Dynamoelectric machine



July 29, 1958 J. L. OLDENKAMP ET AL 2,845,553

` DYNAMOELECTRIC MACHINE Filed Oct. 1l. 1955 United States Patent O DYNAMOELECTRIC lMACHINE 'John L. Oldenkamp and Fred W. Suhr, Fort Wayne, Ind.,

`assignors to General Electric' Company, a corporation of New York Application October 11,1955, Serial' No.` 539,766

1 Claim. (Cl. S10-172) This invention relates to alternatingcurrent dynamoelectric machinery, and more particularly toi reversible `shaded-pole induction motors.

There are many applications, particularly in thesmall motor lield, where shaded-pole inductionmotors provide great overall economy. Such motorsgenerally include a salient pole stator with at least part of the poles yeach having a shading coilembracing a portion thereof to provide a iiux which lags the ux in the main portion of the pole. This phenomenon tends to produce a ro- Itating field and thereby provide a starting torque for the motor. At the same time, however, the area spanned by the shading coil will provide a lower Yflux than the remainder of the pole. This has the result that, in the range of a practical design, an increase inv starting torque by increasing the shaded pole area mustalways involve va` lower running efficiency. This condition necessitatesa `compromise -between starting torque and running efficiency in the design of such motors.

In addition to the diliculties created by the compromise between etiiciency and starting torque, shaded- Apole motors have also had the disadvantage that a strong third harmonic iield is created due to the distributionv of flux in the radial air gap. This third harmonic eld attains synchronous speed at one-third the synchronous speed of the motor, and a little above that speed the field will act as a generator-and will have a `plugging effect which considerably reduces the torque available for the load. It is, therefore, highly desirable to decrease to a great extent the dip in torque which has heretofore been caused by exceeding the synchronous speed of the Vthird harmonic field. Where the motor load is such as a fan orV blower (a common iield of use for'shade'd-pole induction motors), the load will increase as the speed increases due to the -air resistance. In such a case, a strong -third harmonic dip in torque may well cause the'torque available at that point to go below the load; in such-a case, it is impossible for the motor to come upy to "any speed past the point of the third harmonic dip.

Another undesirable feature which has beenmostdifl'icult to avoid in the past has been an unusual amountof noise when a load such as a fan was-operated by a shaded-pole motor. It is highly desirableto eliminate the excessive noise which has heretofore been present Vduring the starting of shaded-polemotors in connection with fan-type loads.

In those cases where only a single direction of` rotation is required, the structure set forth in application Serial l`No.`503,192 by John L. Oldenkamp and Lewis C.

Bolyard, assigned to the assignee of the present application, now abandoned, provides a desirable solution to the "problem of achieving relatively high efficiency and low starting noisewhile decreasing lthe undesirable third harmonic dip compared to most previous shaded-pole structures. However, the solution arrived at in that application ceases to be available when it becomes necessary to provide a reversible shaded-pole induction motor; such sistance.

2,845,553 Patented July 29, 1958 rice an improved reversible shaded-pole -induction -m'otor structure having the desirable features set forthabove. yFurther objects and advantages-of this invention-will become apparent and the invention will be better understood by reference to the following `descriptionand-the accompanying drawing, and the lfeatures lofV` novelty which characterize this invention will be pointed out with -particularity in the claim annexed to and forming a part of this specification. i

yThis invention, in one embodimentvthereoi yprovide an alternating current reversible induction motor oflthe shaded-pole type having a squirrel-cage rotor'with-Ia stator surrounding the rotor and concentric therewith.

`The stator is divided into a plurality'of equispaced salient 5 poles which form radial airxgaps with therotor. -Each `of the poles has a shaded section ateach-end. vA pair of shading coils are respectively arranged'on .the-sectionsof eachpole Aand'are connected so as to be alternatively short circuited. Each pole has its `face chamf'e'red overspans of at least 30 electrical degrees tofeachltip thereof. soas to provide a radial air Igap which increases -continuously over thecharnfered spans as etherltip'fof each pole is approached.

In the drawing, Figure l is'an end'vi'ew, partlyfc'zutv away, of a reversible shaded-pole inductionfmotorprovided with the improved construction of this invention; and

Figure 2 is a graph settingforththe torque-to-speed performance of the improved -motor -`ofthis invention.

"Referring now to Figure lof the drawing,lthere is illustrated a reversible shaded-pole induction motor, generally indicated at 1, having astator y2*and-a"rotor-iii, with the rotor being mounted concentricallyJa-ndrotatably within the stator 1 by any desired meansllnot shown). Stator 2 is preferablyformed of afpluralityof thin laminations of magnetic material (only one lamination is seen in Figure l) which may bev secured together by any desired means .such as rivets 4, -for example. Stator 2 is formed with'six salient poles 5`whi`ch1are wound with a singlev phase running winding 6in1 standard fashion. Rotor 3 is mounted on a shaft 10 and islforiried of a plurality of laminations of .magnetic-rnaterialfhaving openings 7 each containing a-bar 8 of conductivelmateriaL The barsS are-connectedvtogether at eachiendby la ring 9 of conductive material so'thatla "plurality'of closed conducting loops arev provided larouridthesurface of rotor 3.

yThe face of each pole 5 is divided `into apair `offshadd polesections 11 and 12 andfacentral sectionl1'3. "Sections 11 and v1.2 are respectively separated'fromsction 13 by slots 14 and 15 which accommodate respectively wound shading coils 16 and 17. All the coils lirare interconnected, as shown at 18, and `all thecoilslT-*are similarly connected, Vas shown at 19. Coilsf116'and-17 are jointly'connected to a line-20 at one end,Y coils/116 being connected toline' 21 at their kother endtfand lcoils 17 being connected to line22 at their other end. Lines 21 and 22 are respectively provided with contactsi23 and 24 adapted to be selectively engaged b'y a contact arm 25.secured to line 20. WhenY contact arm25is inengagement with contact 23, allfthecoils=16 4will-be in a short circuit; on the otherhhandfwhen contact-ar-m 3 25 is placed in engagement with contact 24, all the coils 17 will be placed in short circuit.

The short circuiting of each group of shading coils will cause those coils to modify the flux within the section they encompass so that it lags the ux in section 13. This, in turn causes rotor 3 to start rotating in either a clockwise or counterclockwise direction. Such coils are well-known and may be formed in any preferred manner. In the present invention, the fact that it is necessary to disconnect one set of coils for a given direction of rotation makes a wound construction preferable.

In the present embodiment, each pole face has its face chamfered from the center of section 13 to each of the tips 26 and 27 of the pole. This provides a continuously changing air gap at the end of the pole which is opposite to the shading coil being utilized for any given direction of rotation. 'As is shown in Figure l, the chamfers are such as to cause the air gap between pole 5 and rotor 3 to increase gradually from the center section 13 to each of the tips 26 and 27 of each pole 5. It will be recalled that it has been stated that the flux in a shaded section is of necessity less than that in the unshaded section of a pole. In previous constructions, the high ux of the unshaded section has been continued over to the tip of the pole. However, by the provision of the chamfering of the pole face 5 from the center of section 13 to each pole tip, with a consequent gradually increasing air gap between the pole face and the rotor 3, the ux passing across the air gap will gradually decrease from the center of section 13 to the unshaded tip of the pole. Thus, for instance, if contact arm is moved into engagement with contact 23, the short circuiting of coils 16 will cause a decrease in the flux passing across the air gap between section 11 of each pole and the rotor 3. However, the chamfering of the pole tip will cause a similar decrease in ilux to be observed as the other end of the pole is approached.

kThis decrease of flux as the other tip 27 of the pole' face is approached will tend to balance the decreased flux due to coils 16 which is present at the other end 26 of the pole and to eliminate to a large extent the plugging effect of the third harmonic iield.

Referring to Figure 2, it will be noted that since the difference between the torque and the load on the motor at any given instance provides the acceleration for the motor, the elimination of the third harmonic dip, to the extent shown, will provide for a substantial amount of acceleration all the way up to running speed of the motor. It will be seen that where, as has previously often been the case, the third harmonic dip is permitted to become substantial, it may dip down quite close to the load curve, thereby precluding any substantial amount of acceleration at that point. If, in fact, the torque curve were to be permitted to touch or go below the load curve at that point, the motor would be unable to accelerate to its full speed and would be obliged to remain at approximately one-third synchronous speed. For this reason, the chamfering described above reduces the amount of the third harmonic dip and improves the performance of the motor by providing better distribution of the flux. Experience has further shown that the provision of the chamfer eliminates to a great extent the noise which has frequently prevailed during starting of this type of motor when connected to a load such as a fan for instance.

As explained before, the larger the section encompassed by a shading coil, the greater will be the starting torque at the expense of the running efficiency. The third harmonic dip in most previous motors was so great as to require that the shading coil be relatively large, on the order of 60 electrical degrees, to provide enough torque at the dip to get the motor up to speed. The diminution of the third harmonic dip eliminates the necessity for a large shading coil span and thus permits considerable improvement in the eflciency. Thus, by providing identical chamfers extending from section 13 to each of the tips 26 and 27 of each pole, it has been possible to improve the starting torque characteristics of the motor while also improving the efficiency, and at the same time attaining a motor with less starting noise than was previously the rule. These advantages are obtained regardless of the direction of rotation selected because of the fact that each pole is provided with two chamfers so that one chamfer will always be effective to balance the decrease in flux caused by the shading coil selected by contact arm 25.

It has further been discovered, however, that it is possible to obtain fully all the advantages set forth above only within specified dimensions. The number of electrical degrees covered by each of the shading coil sections 11 and 12 of each pole 5 should be within the range of approximately 2O to 45, with 35 being about the optimum. This range results from the provision of the chamfer opposite the shading coil; it has been found that each chamfer should extend over a span of no less than 30 electrical degrees to achieve the beneficial results of this invention.

The space between the tip 26 of one pole and the opposite tip 27 of an adjacent pole is normally limited as to a minimum by the desirability of being able to Wind coil 6 by passing it between the tips of adjacent poles and by the fact that excessive leakage between poles is not desirable. The practical maximum for the space is determined by the consideration that too large a gap would waste space within the Vmachine with a consequent lessening of the ilux useful for operation of the motor. These considerations are well-known matters of design and are not deemed to constitute any part of the present invention. However, it is pointed out that where the air gap between poles is decreased below approximately 5 electrical degrees an undesirable amount of flux leakage between the poles will be observed and will detract from the motor performance. fIt has further been observed from experimentation that the optimum results of the invention are obtained `when the chamfered parts are such that the air gap between the tips 26 and 27 of the pole and rotor 3 are approximately one and a half to three times the minimum air gap between each pole 5 and rotor 3. It has been discovered that excellent results may be obtained with a six pole machine, as shown, wherein each pole has a pair of chamfered sections as large as possible, that is, each chamfered section lextends to the `center of section 13, a pair of shading coil sections of approximately 35 electrical degrees each, and an interpolar space of about 15 electrical degrees. The air gap under `the pole tip to be provided with the foregoing dimensions has been found to have a most desirable value of approximately twice the air `gap under the center of section 13.

'Such a motor was constructed to the following specications:

With an applied voltage of Volts, the following performance characteristics were noted:

Clockwise:

Running efficiency 31.7 Full load current 4.88 Starting torque (in percent of maximum torque) 35.1

Dip torque (in percent of maximum torque) 53 Starting noise (1 foot from 24 inch fan using appropriate weighting network) db-- 56 Starting noise (1 foot from 24 inch fan using appropriate lweighting network) db 56 These performance gures indicate that vthe overall performance of the motor was considerably improved over that of any known reversible shaded-pole induction motor of comparable size. In addition to the other previously discussed advantages, the full load current required by the motor was found to be considerably lower than in any known previous design of reversible shaded-pole motor. This is most important where, as is frequently the case, such a motor is to be used for air moving purposes in apparatus designed to be installed in the home, since home wiring is frequently taxed to the limit of its capacities by the number of power-consuming devices presently available for use in the home. This result is attributable to the small shading coil span made possible by the motor design of this invention.

While, for illustrative purposes, a six-pole motor has been described, the invention is equally applicable to motors involving any desired number of poles Iand is not to be restricted to the specific embodiment shown.

Thus, while Vthis invention has been explained by describing a particular embodiment thereof, -it will be apparent that improvements and modifications may be made without departing from the scope of the invention as dened in the appended claim.

What we claim as new and desire to secure by Letters Patent of the United States is:

In a reversible induction motor of the shaded-pole type, a squirrel cage rotor, a stator surrounding said rotor and concentric therewith, said stator being divided into a plurality of equispaced salient pol-es which form radial air gaps with lsaid rotor, each of said po'les having a section at each end, each of said sections having a face extending over a span of approximately electrical degrees, a pair of shading coils respectively arranged on said section of each pole and connected so as to be selectively short circuited, each of said poles 'having its face chamfered from the center of the pole to each "tip thereof thereby to provide a radial air gap which increases continuously over said chamfered .span as the tips of each said pole are approached, each chamfered span forming a radial air Igap increasing to a maximum of approximately twice the minimum radial air gap under the center of said pole, the `tips of adjacent poles being arranged to dene a circumferential air gap of approximately 15 electrical degrees.

References Cited in the le of this patent UNITED STATES PATENTS 1,002,718 Marelli Sept. 5, 1911 1,969,981 Janca Aug. 14, 1934 2,066,760 Blamber-g Jan. 5, 1937 2,773,999 Morrill Dec. 11, 1956 FOREIGN PATENTS 691,620 Great -Britain May 20, 1953 

